The skin and eyes are vital barriers against microbial invasion. Their unique structures and resident microbiomes work together to protect us from pathogens. The skin's layers and secretions, along with the eye's tear film, create inhospitable environments for harmful microbes.

Despite these defenses, some microbes can bypass them through various mechanisms. Understanding how pathogens overcome our natural barriers is crucial for preventing and treating infections. Recognizing the signs of skin and eye infections helps in early diagnosis and intervention.

Anatomy and Normal Microbiota of the Skin and Eyes

Anatomical structures for microbial interactions

• Skin
  • Epidermis outermost layer of skin provides physical barrier against microbes (barrier function)
    • Stratum corneum top layer of epidermis consists of dead keratinocytes and lipids (ceramides, cholesterol)
    • Keratinocytes produce antimicrobial peptides (defensins, cathelicidins) and cytokines to combat pathogens
    • Langerhans cells antigen-presenting cells in the epidermis initiate immune responses to invading microbes
  • Dermis layer beneath the epidermis contains connective tissue and immune cells
    • Fibroblasts produce extracellular matrix components (collagen, elastin) that support skin structure
    • Collagen and elastin fibers provide tensile strength and elasticity to the skin
    • Blood vessels supply nutrients and oxygen to the skin and facilitate immune cell migration
    • Sweat glands produce sweat that contains antimicrobial compounds (dermcidin, lactoferrin)
    • Sebaceous glands secrete sebum that maintains skin moisture and has antimicrobial properties (free fatty acids, glycerol)
    • Hair follicles provide a niche for specific microbes (Propionibacterium acnes) and can serve as entry points for pathogens
• Eyes
  • Conjunctiva thin mucous membrane that covers the white of the eye and inner eyelids
  • Cornea transparent, avascular tissue that covers the front of the eye and provides a barrier against microbes
  • Tear film lubricates and protects the surface of the eye
    • Lipid layer outer layer of the tear film produced by the meibomian glands prevents tear evaporation
    • Aqueous layer middle layer of the tear film produced by the lacrimal glands contains antimicrobial compounds (lysozyme, lactoferrin, immunoglobulins)
    • Mucin layer inner layer of the tear film produced by conjunctival goblet cells helps spread the tear film evenly over the eye surface
  • Lacrimal glands produce tears that flush away microbes and contain antimicrobial compounds
  • Meibomian glands secrete lipids that stabilize the tear film and have antimicrobial properties

Microbiomes of skin vs eyes

• Skin microbiome
  • Varies based on skin region due to differences in sebum production, moisture, and temperature
    • Sebaceous sites (forehead, back) have higher proportions of lipophilic bacteria
      • Propionibacterium acnes predominant species in sebaceous areas thrives on sebum
      • Staphylococcus species (S. epidermidis, S. hominis) also abundant in sebaceous regions
    • Moist sites (axilla, groin) favor the growth of bacteria that thrive in humid environments
      • Corynebacterium species (C. jeikeium, C. striatum) predominate in moist areas
      • Staphylococcus species (S. hominis, S. haemolyticus) also present in moist regions
    • Dry sites (forearm, palm) have a more diverse microbiome with lower bacterial density
      • Proteobacteria (Acinetobacter, Pseudomonas) more abundant in dry areas
      • Flavobacteriales (Chryseobacterium, Elizabethkingia) also present in dry regions
  • Influenced by factors such as age (lower diversity in infants), gender (hormonal effects), and hygiene (handwashing, skincare products)
  • Microbiome diversity contributes to skin health and protection against pathogens
• Eye microbiome
  • Conjunctiva and tear film have a relatively stable and less diverse microbiome compared to the skin
    • Coagulase-negative staphylococci (S. epidermidis) most abundant bacteria on the ocular surface
    • Propionibacterium acnes also present in low numbers
    • Corynebacterium species (C. macginleyi) commonly found on the conjunctiva
    • Streptococcus species (S. mitis, S. oralis) part of the normal ocular flora

Microbial defense mechanisms and interactions

• Innate immunity plays a crucial role in protecting skin and eyes from pathogens
• Resident flora on skin and eyes contribute to defense through microbial antagonism
• Mucosal surfaces, including the conjunctiva, have specialized defense mechanisms
• Some microbes can act as opportunistic pathogens under certain conditions

Microbial defense bypass mechanisms

• Skin
  • Penetration through breaks in the skin barrier allows microbes to access deeper tissues
    • Wounds (cuts, scrapes) disrupt the stratum corneum and provide entry points for pathogens
    • Abrasions (friction burns) remove the top layer of the epidermis, making the skin more susceptible to infection
    • Insect bites (mosquitoes, ticks) can introduce pathogens directly into the dermis
  • Invasion via hair follicles and sweat glands allows bacteria to bypass the epidermis
  • Production of enzymes to degrade skin components facilitates microbial invasion
    • Hyaluronidase breaks down hyaluronic acid in the extracellular matrix, increasing tissue permeability
    • Lipases degrade lipids in the stratum corneum, compromising the skin barrier
    • Proteases (collagenases, elastases) degrade collagen and elastin fibers, weakening the skin structure
• Eyes
  • Adherence to corneal or conjunctival epithelium allows bacteria to establish a foothold on the ocular surface
  • Biofilm formation protects bacteria from antimicrobial agents and immune responses
  • Evasion of tear film antimicrobial components enables microbes to survive on the eye surface
    • Lysozyme enzymatically degrades bacterial cell walls
    • Lactoferrin sequesters iron, limiting bacterial growth
    • Immunoglobulins (IgA) neutralize microbes and prevent their attachment to the ocular surface
  • Invasion through compromised corneal epithelium (due to injury or disease) allows microbes to access deeper layers of the eye

Signs of skin and eye infections

• Skin infections
  • Cellulitis bacterial infection of the dermis and subcutaneous tissue
    • Erythema (redness) due to increased blood flow to the infected area
    • Warmth caused by inflammatory response
    • Swelling (edema) resulting from increased vascular permeability
    • Pain due to the release of inflammatory mediators and pressure on nerve endings
  • Impetigo superficial bacterial infection of the epidermis
    • Honey-colored crusts formed by the accumulation of dried serum and bacteria
    • Erythematous base (red, inflamed skin) surrounding the crusts
  • Folliculitis infection of hair follicles
    • Pustules (pus-filled bumps) or papules (solid bumps) centered on hair follicles
• Eye infections
  • Conjunctivitis inflammation of the conjunctiva
    • Redness due to dilation of conjunctival blood vessels
    • Discharge (purulent or watery) caused by increased mucus production and inflammatory exudate
    • Itching or burning sensation resulting from the release of inflammatory mediators
  • Keratitis infection or inflammation of the cornea
    • Corneal opacity (clouding) due to inflammatory infiltrates and edema
    • Pain caused by the exposure of corneal nerves
    • Photophobia (light sensitivity) due to corneal irritation
    • Decreased visual acuity resulting from corneal opacity and irregular surface
  • Blepharitis inflammation of the eyelid margins
    • Eyelid inflammation (redness, swelling) due to the accumulation of bacteria and debris
    • Crusting or flaking of eyelid margins caused by the buildup of oils, dead skin cells, and bacteria